A large portion of the metallic alloys in use today are processed by solidification casting, at least initially. The metallic alloy is melted and cast into a metal or ceramic mold, where it solidifies. The mold is stripped away, and the cast metallic piece is ready for use or further processing. The as-cast structure of most materials produced during solidification and cooling depends upon the cooling rate. There is no general rule for the nature of the variation, but for the most part the structure changes only gradually with changes in cooling rate. On the other hand, for the bulk-solidifying amorphous alloys the change between the amorphous state produced by relatively rapid cooling and the crystalline state produced by relatively slower cooling is one of kind rather than degree—the two states have distinct properties.
Bulk-solidifying amorphous alloys, or bulk metallic glasses (“BMG”), are a recently developed class of metallic materials. This amorphous state can be highly advantageous for certain applications. If the cooling rate is not sufficiently high, crystals may form inside the alloy during cooling, so that the benefits of the amorphous state are partially or completely lost. For example, one risk with the creation of bulk amorphous alloy parts is partial crystallization due to either slow cooling or impurities in the raw material.
Bulk-solidifying amorphous alloys have been made in a variety of metallic systems. They are generally prepared by quenching from above the melting temperature to the ambient temperature. Generally, high cooling rates, such as one on the order of 105° C./sec, are needed to achieve an amorphous structure. The lowest rate by which a bulk solidifying alloy can be cooled to avoid crystallization, thereby achieving and maintaining the amorphous structure during cooling, is referred to as the “critical cooling rate” for the alloy. In order to achieve a cooling rate higher than the critical cooling rate, heat has to be extracted from the sample.
BMG articles are often formed by injection molding and/or die casting of a molten material cooled by thermal contact with a mold or a die. Problems arise, however, due to shrinking of the cooled material. The shrinkage generates a gap between the molten material and wall of the mold, reduces thermal contact there-between, and thus reduces the cooling rate of the molten material. The reduced cooling rate increases the potential for forming crystalline. In addition, the formed article may have undesired surface finishes and/or an underfilled part due to the gap formed between the molten material and the wall of the mold. Further, it is difficult to form BMG articles with high aspect ratio or small sections. This is because the molten material will cool off so rapidly that it will solidify before it can fill the entire mold cavity.